Broadband waveguide to coaxial line transition

ABSTRACT

Transition apparatus for interconnecting a waveguide to a coaxial line comprising a first subassembly which forms the top, both side and end closing walls of a rectangular waveguide section and a second subassembly which provides the bottom wall thereof and has an impedance transforming section integrally formed therewith. This transforming section is in the form of a staircase and the first and top step thereof has an overhanging tab which confronts the end closing wall of the waveguide section. The center conductor of a coaxial connector attached to this end wall contacts the end face of the tab, and these elements together with the back of the staircase and an adjacent portion of the bottom wall define a magnetic field coupling loop.

United States Patent 1 Gaudio et a1.

[54] BROADBAND WAVEGUIDE TO COAXIAL LINE TRANSITION [75] Inventors: Joseph G. Gaudio, Jefferson Station; Thomas R. Debski, 23 Rose St., Bethpage, both of N.Y.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: Sept. 8, 1972 [21] Appl. No.: 287,320

[451 June 5,1973

Primary Examiner-Rudo1ph V. Rolinec Assistant ExaminerSaxfield Chatmon, Jr. Attorney R. S. Sciascia and L. l. Shrago 57 ABSTRACT Transition apparatus for interconnecting a waveguide to a coaxial line comprising a first subassembly which forms the top, both side and end closing walls of a rectangular waveguide section and a second subassembly which provides the bottom wall thereof and has an impedance transforming section integrally formed therewith. This transforming section is in the form of a staircase and the first and top step thereof has an overhanging tab which confronts the end closing wall of the waveguide section. The center conductor of a coaxial connector attached to this end wall contacts the end face of the tab, and these elements together with the back of the staircase and anadjacent portion of the bottom wall define a magnetic field coupling loop.

6 Claims, 3 Drawing Figures Pate nted June 5, 1973 FREQ (6H2) Fig.3

W4 DE q 19 Fig.2

BROADBAND. WAVEGUIDE TO COAXIAL LINE TRANSITION the end portion of the inner conductor of the coaxial line terminate on one broad wall of the waveguide so as to coact with the electric field present in this region of the waveguide. With this electric field type of coupling, the coaxial transmission line projects from the waveguide at right angles, and this geometry sometimes is not desirable where only limited space is available for making this connection.

The transition apparatus may, of course, utilize the magnetic field associated with the electromagnetic wave energy propagating in the waveguide. If the inner conductor of the coaxial transmission line is utilized as a probe to couple to this magnetic field, then the longitudinal axis of the coaxial line may be aligned with the propagation axis of the waveguide. With such an orientation of axes, the overall structure requires less space than those depending upon electric field coupling.

It is accordingly a primary object of the present invention to provide a broad-band waveguide to coaxial line transition utilizing magnetic field coupling.

Another object of the present invention is to provide a transition between waveguides and coaxial transmission lines which permits the longitudinal axis of the coaxial line to be aligned with the propagation axis of the waveguide.

Another object of the present invention is to provide a waveguide to coaxial line transition of simplified and compact construction.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates a transition arrangement according to the present invention interconnecting a rectangular waveguide section and a coaxial line;

FIG. 2 shows some of the pertinent dimensions of the apparatus; and

FIG. 3 is a plot of the voltage standing wave ratio versus frequency for one practical embodiment of the invention.

Referring now to FlGfl of the drawings which illustrates a relatively simple embodiment of the invention, it will be seen that the overall transition apparatus consists of two major subassemblies. The first, lower section 10, forms the bottom broad wall of a rectangular waveguide section and includes as an integral element thereof an impedance matching section 11. The secand, upper section 12, forms the top, side and end closing walls of this same section.

A suitable coaxial connector 14 is secured to end wall 13 by any appropriate fastening means. The center pin 15 of this connector is arranged to extend into the interior of the waveguide-section and contact the end face 16 of tab 17 which, as will be seen hereinafter, is the last and top step ofimpedance matching section 11.

- protect the interface. Coaxial connector 14 also includes as acomponent thereof a Teflon loading portion 18 which surrounds pin 15 and, like this pin, extends into the interior of the waveguide section and also terminatesat face 16.

In order to match the usually higher impedance of a waveguide section to a coaxial line, the general practice is to decrease the narrow dimension of the waveguide, that is, the distance between the broad walls of a rectangular waveguide in a series of steps so as to arrive at an internal dimension that achieves an acceptable impedance match with a satisfactory voltage standing wave ratio. This approach is utilized in the present apparatus, and the complete impedance transformer 11 consists of four quarter wave sections. These sections take the form of a staircase of individual steps 19, 20, 21 and 22 with the first and top step 22 having an overhanging tab 17. The heights of the steps which are generally unequal, are chosen in accordance with a set of numerical coefficients referred to as Tchebyscheff coefficients. The back corners of the end face 16 of this tab are chamfered to reduce the capacitance between the end of this tab and the back wall 13 of the waveguide assembly. In order to further simplify the construction of the transition apparatus, section 11 is formed as an integral part of the lower subassembly 10.

The magnetic coupling loop of the apparatus is formed by center pin 15 of the coaxial connector, tab 17 in contact therewith and the back vertical wall portion 23' of the third step of the staircase which connects with the bottom broad wall of the waveguide. Center pin 15 in this regard is not necessarily centered vertically on back wall 13. Tab 17, back wall 23 and the adjacent portion of the bottom broad wall in combination appears as a quarter wave short circuited stub connected in parallel at the junction between center pin 15 and end face 16 of tab 17.

Referring now to FIG. 2 of the drawings, it would b pointed out that the dimension BC, the distance between the back wall 13 and the end surface 16, in actual practice may be between one-hundredth and onetenth of a wavelength depending upon the amount of capacitance required. The dimension DE, the distance between the back wall surface 23 and end wall 13, is approximately one-fourth of a wavelength. The width of each step is generally between one-third and onefourth of the total width of the broad wall of the waveguide. Dimension HI between the top broad wall and the top surface of tab 17 is chosen to achieve the proper impedance level in the first transformer section. It would be appreciated that this impedance level in turndepends on the impedance of the particular coaxial line and the particular waveguide which are to be interconnected by the transition apparatus.

The short circuited stub, above mentioned, which forms part of the magnetic coupling loop will present excess inductance at the low end of the frequency band, and excess capacitance at the high end of this band. If the number of transformer sections are even, the net contribution of the transformer section will be an excess capacitance at the low end ofthe frequency band and an excess inductance at the high end thereof. This is generally true since, in most cases, the waveguide s impedance is higher that the coaxial line impedance. Consequently, if the number of transformer sections is even, as is the case illustrated, the reactive effects contributed by the stub and by the transformer, respectively, will tend to cancel out. In the unlikely event that the waveguide impedance were lower than the coaxial line impedance, it would be preferable if the number of transformer stages were odd.

The impedance levels of the various transformer sections are generally selected on an analytical basis, however, it may be noticed that either an excessive inductive s'usceptance or capacitative susceptance appears at the junction between pin and tab 17. If a small excess capacitative susceptance is observed, this may be remedied by a small cut taken from surface I6 of tab 17 which increases the dimension BC. If a small excess inductive susceptance is observed, this may be remedied by a small cut taken on the underside of the staircase from surface 23 which increases the dimension DE.

At the waveguide end of the transition the similar dispersion characteristics of the rectangular waveguide and the ridge waveguide due to the step construction tend to minimize any junction reactance. At the coaxial end, the ridge loading is such that a quasi-TEM or microstrip mode is set up. Most of the electric field is confined to the region of the ridge and a smooth transition to the coaxial line is accomplished.

The embodiment depicted in FIG. 1 was developed for an impedance ratio of about 8 to l and a frequency range of 7 to l I GHz. FIG. 3 is a plot of voltage standing wave ratio (VSWR) versus frequency for a transition between a 0.400 X 1.020 inch waveguide and a co- .axial line. The unit which used four transformer stages,

as described above, was 1.700 inches long. The connector type was 3mm. Over a frequency band of 7 to 11 GHz, the VSWR as shown was less than l.28:l.

What is claimed is: 1. Apparatus for interconnecting a waveguide to a coaxial transmission line comprising, in combination,

a rectangular waveguide section closed off at one end thereof by an end wall and adapted to be connected to said waveguide at the other end thereof, said section being formed by a first unitary member which serves as the bottom wall of said waveguide section and a second unitary member which serves as the other three walls and the end wall of said waveguide section;

an impedance transformer integrally formed with said bottom wall, said transformer consisting of a staircase of attached steps with successive steps being progressively higher as said steps occur at nearer distances to said end wall and including a top step which has an overhanging tab with the rear vertical face thereof being parallel to and spaced a preselected distance from said end wall;

a coaxial connector mounted on said end wall with the center pin thereof extending into the interior of said waveguide section and terminating at said rear vertical face of said overhanging tab, said center pin, said tab, the rear common face of the other steps and an adjacent surface portion of said bottom wall forming a magnetic field coupling loop and the space between the rear face of said tab and a confronting surface area of said end wall forming a capacitive susceptance for tuning purposes.

2. In an arrangement as defined in claim 1 wherein said staircase has an even number of steps.

3. In an arrangement as defined in claim 2 wherein the rear commonface of the other steps is parallel to said end wall and spaced therefrom by a distance corresponding to one-quarter wavelength of the electromagnetic wave energy that it normally propagating within said waveguide.

4. In an arrangement as defined in claim 1 wherein said steps have a width that is between one-quarter and one-third of the total width of the bottom wall of said rectangular waveguide section.

5. In an arrangement as defined in claim 1 wherein the back corners of said overhanging tab are chamfered off so as to reduce the capacitance between said tab and said back wall.

6. In an arrangement as defined in claim 1 wherein said coaxial connector includes a dielectric member which surrounds said center pin and extends into the interior of said rectangular waveguide section, terminating near the rear vertical face of said overhanging tab. 

1. Apparatus for interconnecting a waveguide to a coaxial transmission line comprising, in combination, a rectangular waveguide section closed off at one end thereof by an end wall and adapted to be connected to said waveguide at the other end thereof, said section being formed by a first unitary member which serves as the bottom wall of said waveguide section and a second unitary member which serves as the other three walls and the end wall of said waveguide section; an impedance transformer integrally formed with said bottom wall, said transformer consisting of a staircase of attached steps with successive steps being progressively higher as said steps occur at nearer distances to said end wall and including a top step which has an overhanging tab with the rear vertical face thereof being parallel to and spaced a preselected distance from said end wall; a coaxial connector mounted on said end wall with the center pin thereof extending into the interior of said waveguide section and terminating at said rear vertical face of said overhanging tab, said center pin, said tab, the rear common face of the other steps and an adjacent surface portion of said bottom wall forming a magnetic field coupling loop and the space between the rear face of said tab and a confronting surface area of said end wall forming a capacitive susceptance for tuning purposes.
 2. In an arrangement as defined in claim 1 wherein said staircase has an even number of steps.
 3. In an arrangement as defined in claim 2 wherein the rear commonface of the other steps is parallel to said end wall and spaced therefrom by a distance corresponding to one-quarter wavelength of the electromagnetic wave energy that it normally propagating within said waveguide.
 4. In an Arrangement as defined in claim 1 wherein said steps have a width that is between one-quarter and one-third of the total width of the bottom wall of said rectangular waveguide section.
 5. In an arrangement as defined in claim 1 wherein the back corners of said overhanging tab are chamfered off so as to reduce the capacitance between said tab and said back wall.
 6. In an arrangement as defined in claim 1 wherein said coaxial connector includes a dielectric member which surrounds said center pin and extends into the interior of said rectangular waveguide section, terminating near the rear vertical face of said overhanging tab. 